Experimental animals (guinea pigs and Old World monkeys) are trained by operant conditioning procedures with food reinforcement on one or more of several protocols for hearing evaluation. In addition to pure tone audiometry, measures of loudness recruitment, differential thresholds for frequency and intensity, psychophysical tuning curves, critical bands, and discrimination of complex signals such as speech are proposed. When normal behavioral baselines are stable following training, the animals are treated with one of the aminoglycoside antibiotics or exposed to intense sound until substantial, although not complete, hearing loss has occurred. After treatment with drugs or noise the hearing assessment is continued until the behavioral measures are stable for at least one month. Animals are then sacrificed and their temporal bones taken for microdissection and analysis of the cochlea by both light and electron microscopy. It is hoped that the observed relation between missing hair cells in the cochlea and the behavioral measures will reveal information regarding the function of these receptor cells.